1. Field of the Invention
The present invention relates to a piezoelectric filter, especially intermediate frequency filter or the like, utilizing a piezoelectric resonator, mainly used in radio appliances such as mobile communication appliances, acoustic appliances, and others, and its manufacturing method.
2. Related Art of the Invention
Generally, as intermediate frequency filter used in radio appliances or the like, the piezoelectric filter making use of contour-extensional mode (radial mode) of a disk or square plate piezoelectric resonator is employed in a wide range. That is, these piezoelectric resonators are composed in a ladder structure, the dimension of the series elements and the dimension of parallel elements are adjusted, the resonance frequency of series elements and antiresonance frequency of parallel elements are matched to compose a passing region, and attenuation poles are formed at both sides, so that a steep filter characteristic is realized. At this time, the matching impedance of the filter is the matching impedance at the center frequency of the filter.
To realize a steep filter characteristic, each piezoelectric resonator must be supported at a position of a node of vibration of the vibration mode to be utilized in order to avoid vibration blocking. For this supporting, hitherto, the pressure between a metal leaf spring and case for package was utilized, or instead of the leaf spring, directly, electrode plates for input, for output and for earth were used to make use of the pressure against the case. At this time, the piezoelectric resonators are wired by leaf spring for wiring which is composed by preliminarily integrating the necessary electrode plates to be connected.
Referring now to the drawings, an example of such conventional piezoelectric resonator is described below.
FIG. 46 is a perspective exploded view showing a constitution of a ladder type piezoelectric ceramic filter making use of a square plate contour-extensional mode (radial mode) disclosed, for example, in Japanese Laid-open Utility Model No. 4-114229. FIG. 47 is a circuit diagram showing its equivalent circuit. In FIG. 46, reference numeral 321 is a square plate type piezoelectric resonator for series elements (hereinafter called piezoelectric resonator), and 322 is a square plate type piezoelectric resonator for parallel elements (hereinafter called piezoelectric resonator). These piezoelectric resonators 321, 322 are different in thickness and electrode area for the purpose of adjustment of electric capacity. Reference numeral 323 is an electrode plate for input, 324 is an electrode plate for output, 325 is an electrode plate for earth, and 327 is an electrode plate for wiring. In the electrically connecting portion of these electrode plates 323 to 325, 327 and the piezoelectric resonators 321, 322, an emboss 326 is formed in order to support the piezoelectric resonators 321, 322 securely at the position of a node of vibration. Moreover, reference numeral 328 is a package, and 329 is a leaf spring for applying pressure for support of the piezoelectric resonators 321, 322. Reference numeral 330 is a sealing plate to be adhered to the package after inserting the piezoelectric resonators 321, 322, electrode plates 323 to 325, 327, and leaf spring 329 into the package.
In such conventional constitution, however, since the central portion of the piezoelectric resonators 321, 322 at the node of vibration is supported by press-fitting with the electrodes plates 323 to 325 and 327 by the pressure of the leaf spring 329, assembling is difficult, and the supporting position of the piezoelectric resonators 321, 322 is deviated by external impact, the resonance characteristic of the piezoelectric resonators 321, 322 varies, the filter characteristic fluctuates, and a large change occurs in the time course.
Or, in the case of ladder type wiring by electrode plates, plural types of electrode plates 323, 324, 325, 327 are needed, and therefore the number of parts is many, the assembling process is complicated, and the cost is increased.
Besides, using a square plate type piezoelectric resonator, the shape of the piezoelectric resonator is large, and when the number of stages of the filter is increased, the filter size is extremely increased.
Structurally, furthermore, since assembling from one direction is difficult, and it is hard to automate the assembling, and mass production is difficult.
On the other hand, recently, as the mobile communication field is digitized, not only the attenuation characteristic is important, but also the group delay time characteristic comes to be regarded with special importance. Herein, the group delay time is obtained by differentiating the phase by frequency. Meanwhile, since the digital signal is modulated and demodulated by deviation of phase, accurate modulation or demodulation is not guaranteed unless the phase rotation changes linearly. For accurate modulation and demodulation, accordingly, the group delay time characteristic is utilized as the index for expressing the shift of the phase rotation from linearity by the time difference.
Referring further to the drawings, a conventional method of flattening the group delay characteristic in a required band of a piezoelectric ceramic filter having a basic structure shown in FIG. 46 is described below.
FIG. 48 shows the filter characteristic of a conventional piezoelectric filter, and FIG. 49 is a filter characteristic diagram for explaining a conventional design technique for flattening the group delay characteristic within a specific band. The solid line in FIG. 48 shows the attenuation characteristic, and the broken line denotes the group delay characteristic. In FIG. 48, the attenuation characteristic outside the passing band is extremely moderate, and hence the selective characteristic is insufficient in the portion of large attenuation amount. This is because, in order to flatten the group delay characteristic in the specific bandwidth, the bandwidth is designed broad by employing the vibration mode of large electric-mechanical coupling factor or material of large coupling factor, thereby setting preliminarily large the frequency region in which the group delay characteristic is flat, and the mechanical quality Qm is lowered by the damping resistance or conductive rubber, and the bandwidth is narrowed to the desired bandwidth while keeping large the flat region of group delay characteristic. Accordingly, the insertion loss also increases.
The above design technique is specifically described below by reference to FIG. 49.
In FIG. 49, broken line 1 denotes the attenuation characteristic in an initial design, and broken line 2 indicates the group delay characteristic at this time. Herein, the initial design bandwidth is set larger than the specific bandwidth, and therefore the portion of large group delay time is positioned at both remote ends of outside of the specific bandwidth as seen from the center frequency. In this state, the group delay characteristic in the specific bandwidth is nearly flat. By lowering Qm of the resonator in this state, the attenuation characteristic of solid line 1 is realized and a desired bandwidth is obtained. At this time, the group delay characteristic is as shown by solid line 2, and the peak value at both sides of the group delay time is small, and the flat portion of the group delay characteristic is not narrowed. Techniques for lowering the Qm of the resonator include a method of inserting a conductive rubber 331 between electrode plate and piezoelectric resonator as shown in FIG. 50 (for example, Japanese Laid-open Utility Model No. 4-114229), a method of connecting a damping resistance R332 to each resonator as shown in FIG. 51 (for exampler Japanese Laid-open Utility Model No. 4-121124), and a method of using directly a piezoelectric material with small Qm.
In other design techniques, the value of Qm of each piezoelectric resonator of series and parallel branches of input and output ends for composing the filter is set more than 1.5 times the Qm value of the piezoelectric resonator in each intermediate position (for example, Japanese Laid-open Patent No. 1-314008), the difference .DELTA.f between resonance frequency and antiresonance frequency of each piezoelectric resonator is varied and the arrangement is changed (for example, Japanese Laid-open Utility Model No. 1-091328), or the capacity ratio of piezoelectric resonators is changed (for example, Japanese Laid-open Patent No. 54-163649), and thus it has been attempted to improve the attenuation characteristic and group delay characteristic by considering the material and manufacturing process.
In such conventional methods of improvement however, if the group delay characteristic is improved by designing large the filter bandwidth, the design method for assuring the symmetricity of the filter characteristic is not established, and the selective characteristic outside the specific bandwidth is not achieved, and the design technique has its own problems. To obtain a flat group delay characteristic, by using conductive rubber or damping resistance, the number of parts increases and assembling is difficult. Moreover, to vary the Qm of the resonator or frequency difference .DELTA.f of resonance frequency and antiresonance frequency, plural piezoelectric materials are used, or piezoelectric resonators of partial electrodes or unsaturated polarization are needed, and hence the number of parts increases, the process is complicated, and stable piezoelectric characteristic is not obtained. In any case, by the lowered portion of Qm (the portion of adding the resistance), the insertion loss of the filter increases. In the conventional constitution, only the peak value of the group delay ripple deviation is lowered by adding the resistance, and nothing has been discussed about the symmetrical constitution of the group delay characteristic with respect to the center frequency.
On the other hand, a principal problem in digital signal transmission technology is increase of code errors by the inter-code interference the pulse receives, derived from the distortion of the pulse waveform generated when the pulse signal passes through the transmission path.
Usually, the frequency spectrum of square pulse theoretically has an infinite frequency component. Therefore, in order to receive accurately the square pulse waveform transmitted through the transmission path, theoretically, it is required that the transmission path can transmit an infinite frequency component. Accordingly, an extremely wide band is needed in the frequency characteristic for the transmission signal in the transmission path, and it is not desired from the viewpoint of utilization efficiency of the transmission frequency band, and, in addition, an unnecessarily extra noise portion is also received.
When the transmission path band is narrowed to decrease this noise component, the reception pulse waveform is expanded in the time axis direction, and an adverse effect is exerted on the distinguishing point of the front and rear adjacent pulses. Therefore, it is desired to equalize into pulse waveform small in the inter-code interference and small in noises, that is, minimum in the code error rate.
The most basic band limiting is the use of filter having an ideal low frequency filter characteristic. The response when an impulse is applied to this filter is a known response waveform 151 as shown in FIG. 52.
In this response waveform 151, excluding the central peak at t=0. zero point appears in every To (=1/2 of). In this case, interval To until t=0 to To is called a Nyquist interval 152, and by transmitting the impulse row at this Nyquist interval 152, inter-code interference can be completely avoided for momentary detection done in the middle of the reception pulse. The pulse row of smaller interval than this Nyquist interval 152 is not transmitted usually because the fundamental wave component is cut off. That is, the bit rate corresponding to the Nyquist interval 152 gives the transmission limit in its band.
It is actually extremely difficult to realize such ideal low frequency filter characteristic. Accordingly, to determine the filter condition in actual transmission path, the first standard of Nyquist is used.
According to the first standard of Nyquist, as shown in FIG. 53a, regarding the cut-off angular frequency .omega.o, a filter characteristic 162 of an odd-symmetric filter having an odd-symmetric property is added to a filter characteristic 161 of an ideal filter to synthesize, and when a filter characteristic 163 as shown in FIG. 53b is formed, the intersecting point on the axis of abscissas (.omega.to) of an impulse response waveform 165 is not changed as shown in FIG. 53d.
The filter characteristics shown in FIG. 53b and FIG. 53c can be realized, and it is characteristic that there is no inter-code interference to the impulse transmission of repeated frequency 2fo. There are infinite odd-symmetric frequency characteristics that can be added to the filter characteristic 161 of ideal filter, and hence there are also infinite synthesized filter characteristics made by such adding. Of such synthesized filter characteristics, the filter characteristic generally used widely is the filter characteristic 163 shown in FIG. 53b, which is called roll-off spectrum, and is expressed in formula 1. ##EQU1## where .alpha. is a coefficient expressing the degree of inclination of band limiting, called the roll-off factor, and by varying the value of the roll-off factor .alpha. in a range of 0.ltoreq..alpha..ltoreq.1, as shown in FIG. 53c, the filter characteristic can be changed from the filter characteristic 164a at .alpha.=0 to the filter characteristic 164b at .alpha.=1. Besides, .omega.o is expressed as .omega.o=.pi.To by using the Nyquist interval To.
Usually, the combination of root roll-off filter characteristics by distributing the characteristic expressed in formula 1 equally into the transmission side and reception side of the transmission system is the optimum transmission system characteristic for minimizing the code error rate.
To visually observe the reception pulse waveform including the inter-code interference and noise, an eye diagram (eye pattern) schematically depicted in FIG. 54 is convenient. The eye diagram is a waveform depicted on an oscilloscope, by demodulating a digital signal row modulated by a random pulse at the reception side, and synchronizing the demodulated waveform by clock pulse.
The opening portion 171 of a waveform pattern looking like an eye near the recognition point of the eye diagram shown in FIG. 54 is called the eye, and the allowance of code discrimination can be determined from this aperture rate. Supposing the aperture rate of the eye has deterioration of .DELTA.V in the amplitude axial direction (vertical direction) and .DELTA.T in the time axial direction (lateral direction), as compared with the ideal eye, the eye aperture rate in the amplitude axial direction is expressed as V.sub.a /V.sub.p, and the aperture rate in the time axial direction is T.sub.a /T.sub.p, and the quality of the transmission system characteristic can be evaluated by these eye aperture rates.
This evaluation by eye aperture rate is convenient as qualitative observation because it intuitively appeals to the human senses, but it lacks quantitative evaluation.
On the other hand, as a quantitative evaluation scale of digital signal transmission system, the vector amplitude error expressing an error from an ideal modulation waveform is known. It is, as shown in FIG. 55, a vector amplitude error 182 expressing a deviation from an ideal signal 181, on the basis of the coordinate R of amplitude/phase of ideal .pi./4 shift QPSK signal, and coordinate S of amplitude/phase of actually measured .pi./4 shift QPSK signal.
Generally, the ratio of (absolute value of vector amplitude error 182)/(absolute value of amplitude of ideal signal 181) is called the modulation precision, and it is used as the evaluation scale in the digital signal transmission system. In the RCR (Research & Development Center of Radio System) standard, the modulation precision of transmission signal is specified to be within 12.5%.
Thus, in the digital transmission system, it is important to realize the transmission path capable of obtaining a response of small code error by eliminating the inter-code interference to the impulse signal, and the roll-off filter is known as a most representative filter capable realizing such impulse response. By using this roll-off filter as the intermediate frequency filter in a communication appliance in digital radio transmission system, the code error rate derived from inter-code interference can be reduced also in the digital radio transmission system.
The roll-off filter suited to digital signal transmission mentioned so far can be, of course, composed easily by using individual parts such as inductors and capacitors, and has been used conventionally. Such LC filter, however, composed by using inductors (L) and capacitors (C) has its limit in downsizing, and cannot satisfy sufficiently the demand of the days of miniaturization of communication appliances, and it is already a past item.
Accordingly, ever since piezoelectric properties are recognized in ceramic elements made of barium titanate (BaTiO.sub.3), piezoelectric ceramic materials becoming capable of selecting freely the dielectric constant, piezoelectric properties, mechanical quality coefficient, temperature characteristic and others as a result of development of composition called three-component PZT come to be used widely as material for resonators owing to many features as compared with other piezoelectric materials, such as large piezoelectric property, large dielectric constant, wide range of characteristic obtained by varying the composition, free forming of shape, high stability, mass producibility, low cost, suitability to downsizing, and many others.
Therefore, this piezoelectric ceramic material can be used as the material for intermediate frequency filter in 450 Hz band used in a digital mobile communication appliance, and hitherto ceramic filters composed by using piezoelectric ceramic materials have been widely used as intermediate frequency filters in digital mobile communication appliances.
The conventional intermediate frequency filter composed of such ceramic filter has been used widely for the purpose of downsizing in the mobile communication appliances in digital radio transmission system, and is presently used as intermediate frequency filter, but in this ceramic filter, assuming the deviation of frequency by temperature characteristic and the like, the 3 dB passing band of .+-.10.5 kHz is designed somewhat wider, and a resistor is externally fitted to the resonator in order to decrease the group delay deviation, and the modulation precision of the required limit is achieved by such means in most cases. When using the filter of such kind in the digital transmission system, at the present, there is almost no information useful and practical for its design and manufacture, and, although the size can be reduced, the design is difficult and manufacture is not easy.
In general radio frequency signal transmission system such as digital signal transmission system using a radio signal frequency signal, such a ceramic filter is generally used only at the reception side as an intermediate frequency filter, and therefore in the digital radio transmission system, owing to the above problems, it was extremely difficult to realize the root roll-off characteristic at the reception side where the intermediate frequency filter composed of such ceramic filter is used.